1. Technical Field
The present invention relates to a method and apparatus for extruding plural-component synthetic fibers in a spin pack, and to a multi-component fiber so produced as to be separated into multiple individual fibers. More particularly, the present invention relates to an improved polymer melt/solution spinning method and apparatus permitting a wide variety of plural-component fiber configurations to be extruded at relatively low cost, with a high density of spinning orifices, and with a high degree of fiber uniformity.
2. Discussion of the Prior Art
For certain applications it is desirable to utilize a melt or solution spinning system to extrude tri-lobal shaped bi-component fibers wherein only the three tips of the fiber lobes are of a different polymer from the central core of the fiber. In my prior U.S. Pat. No. 4,406,850, there is disclosed a spin pack which extrudes sheath-core bi-component fibers. For purposes of general reference and an understanding of the state of the art, the disclosure in that patent is expressly incorporated herein, in its entirety, by this reference. If that pack is utilized with a tri-lobal type spinneret, tri-lobal fibers are provided with a coating of the sheath fiber entirely around each fiber periphery. This is not, however, the same as having the tips of the tri-lobal configuration made of sheath polymer. To achieve only tip coverage by sheath polymer, it is necessary to create four separate streams of polymer in laminar flow within the counterbore or inlet hole of each spinneret orifice. A three-legged slot at the downstream end of the orifice would then issue a fiber of the required configuration. One might consider using the same spin pack design and melt spinning method described in my aforesaid U.S. Pat. No. 4,406,850, modified by incorporating three notches cut into the buttons surrounding each spinneret inlet hole and by deleting the spacer shim. These equally spaced notches would allow the sheath polymer to pass through the added notches so as to combine with the core polymer, resulting in the desired four streams of polymer in the spinneret inlet holes and producing the desired type of fiber. For two reasons, this method and the apparatus are not altogether satisfactory. For efficient production, it is desirable to have about eight or so spinning orifices in each square centimeter of spinneret face area, to thereby provide approximately four thousand holes in a rectangular melt spin pack of manageable size. Further, it is desirable to have the spinning orifices positioned in staggered rows for best fiber quenching. The spin pack illustrated in my aforesaid patent is not appropriate for either of these requirements. Specifically, since core inlet holes must be drilled through a rib of metal lying between sheath polymer slots, the rib of metal is limited as to how thin it might be. I have successfully put these ribs on eight millimeter centers; the inlet holes can be drilled on centers spaced by approximately 2.5 millimeters, permitting twenty square millimeters per orifice, or a maximum density of five orifices per square centimeter. Furthermore, my prior patented spin pack requires that the orifices be arranged in straight rows, not staggered, in order that the core polymer holes can be drilled through the straight metal ribs.
It is also desirable to extrude very fine fibers for some applications. Short irregular fine fibers can be made by "melt blowing", or by a centrifugal spinning technique (i.e., cotton-candy machine), or by spinning a blend of incompatible polymers and then separating the two polymers (or dissolving one of the components). All of these techniques produce fibers which are very irregular, vary in denier, and are not continuous for very long lengths. There are known techniques for extruding more uniform continuous fine fibers. For example, U.S. Pat. No. 4,445,833 (Moriki) and U.S. Pat. No. 4,381,274 (Kessler) are typical of fairly recently developed methods of making such fibers. Moriki employs a technique wherein a number of core polymer streams are injected into a matrix or sheath stream via small tubes, one tube for each core stream. Each of Moriki's spinneret orifices produce a fiber with seven "islands in a sea" of sheath polymer. Such a spinneret is suitable for extruding continuous filament yarn with one hundred twenty-six filaments of perhaps 0.3 denier per filament, if the sheath polymer were dissolved away, leaving a bundle of one hundred twenty-six fine core fibers. At 0.3 denier per fiber, the yarn denier would be 37.8, suitable for fine fiber apparel and garments. The Moriki technique is not suitable for extruding large numbers (e.g., 1,000 to 10,000) of multi-component fibers from each spinneret as is necessary for economical production of staple fibers via melt spinning. Even larger number of fibers per spinneret (e.g., 10,000 to 100,000) are necessary for economical wet spinning of polymer solutions. By using tubes to feed each core stream, the number of tubes is limited by the smallest practical size of hypodermic tubing available thereby requiring considerable space. Additionally, if very fine tubes are employed, it would be expensive to assembly them into their retainer plate. In cleaning the spin pack parts (typically, every week), it would be hard to avoid damaging the tubes. Since the tubes have an inside diameter with a very high ratio of length to diameter (i.e., L/D), it would be very hard to clean the inside of each tube. The tube design would certainly make the parts too expensive to be discarded and replaced instead of being cleaned. When clean and undamaged, however, the Moriki device should make very uniform high-quality fibers.
The Kessler apparatus, on the other hand, is more rugged. This apparatus employs machined inserts, permitting a number of polymer side streams to be placed about the periphery of a central stream. Also, by using short tubes (see FIG. 11 of the Kessler patent), some side streams can be injected into the center of the main stream, giving a result which would be similar to that obtained by Moriki. Again, size limitations on the machined insert, and the smallest practical side tubes, make the Kessler apparatus suitable for spinning a limited number of composite filaments per spinneret. Proper cleaning and inspection of the side stream tubes requires removing them from their support plate, a very tedious process for a spinneret with one thousand or more inserts. The Kessler technique may, however, be quite suitable for making continuous filament yarn, as described above for Moriki.
Another class of bi-component or multi-component fibers are being produced commercially wherein the different polymer streams are mixed with a static mixing device at some point in the polymer conveying process. Examples of such processes may be found in U.S. Pat. No. 4,307,054 (Chion) and U.S. Pat. No. 4,414,276 (Kiriyama), and in European Patent Application No. 0104081 (Kato). The Kato device forms a multi-component stream, in the same manner as does Moriki, using apparatus elements "W" shown in FIG. 5 of the Kato disclosure. Kato then passes this stream through a static mixing device, such as the mixer disclosed in U.S. Pat. No. 3,286,992. The static mixer divides and re-divides the multi-component stream, forming a stream with hundreds, or thousands, of core streams within the matrix stream. If the matrix is dissolved away in the resulting fiber, a bundle of extremely fine fibers is produced. Kato also discloses (in FIG. 7 of the Kato disclosure) that a mixed stream of two polymers may be fed as core streams to a second element of the "W" type wherein a third polymer is introduced as a new matrix stream. It should be noted that the apparatus of the present invention, particularly the embodiment illustrated in FIGS. 31-33 of the accompanying drawings, could be used as a less costly and more practical way to construct elements "W" of the Kato assembly.
Kiriyama discloses a method for extruding a fiber assembly that is much simpler than the Kato method, but results in much inferior fibers. The similarity is that Kiriyama employs a static mixer to blend two or more polymers before spinning them into fibers. A wire screen or other bumpy surfaced element is used as the spinneret. The result is that the polymer streams oscillate just prior to solidification, and alternately bond and unbond to each other in a manner to give a bonded fiber structure of primarily fibrous character. Kiriyama does not claim to make very fine fibers; rather, the illustration in FIG. 21 of the Kiriyama patent shows a typical assembly having fibers with an average denier of 2.6, easily attainable by normal melt spinning. Further, since Kiriyama simply blends two streams with the static mixers, and does not initially form "islands in a sea" as does Kato, Kiriyama's fibers are more of a laminar type (see Kiriyama FIGS. 8, 9 and 19), rather than a sheath-core type; some fibers have only one polymer, and in most of them, each polymer layer extends to the periphery of the fiber. The Kiriyama method requires very slow spinning because the fibers must be solidified very close to the screen spinneret; otherwise, all of the streams will simply merge into one large stream. The productivity is quite good due to a high spinning orifice density, but the highest productivity described in the patent is 4.75 gm/min/sq-cm (example 2), and this is no more than is achieved in normal staple spinning of 2.6 denier fibers.
Chion utilizes a technique similar to that of Kato except that Chion employs many closely spaced static mixers, and only one stream of each of the two polymers is fed to the mixer inlets. The equipment is much more rugged and practical than the delicate tubes employed by Kato; however, the resulting fibers are similar to the Kiriyama fibers, laminar in construction rather than "islands in a sea", since Chion starts with two half-moon shaped streams at the top of the mixers and simply divides and re-divides. If the mixed melt is then divided into one thousand or more spinning orifices, one obtains bilaminar and multi-laminar fibers with a few mono-component fibers, but almost no sheath-core fibers.
In addition to high productivity (i.e., grams of polymer per minute per square centimeter of spinneret surface area) and fiber uniformity (i.e., denier and shape), there are other important features that must be considered in devising practical spinning methods. One such consideration is cost, including both the initial purchase price of the spin pack and the maintenance cost therefor. In the prior art described above, all of the polymer distribution plates are relatively expensive, thick metal plates which must be accurately drilled, reamed or otherwise machined at considerable expense. Moreover, with use, polymer material tends to solidify and collect in the distribution flow passages which must be periodically cleaned, and then inspected in order to ensure that the cleaning process has effectively removed all of the collected material. The small size of the flow passages renders the inspection process tedious and time-consuming and, therefore, imparts a considerable cost to the overall cleaning/inspection process. The high initial cost of the distribution plates precludes discarding or disposing of the plates as an alternative to cleaning. In U.S. Pat. No. 3,787,162 (Cheetham) there is disclosed a spin pack for producing a sheath/core conjugate fiber. That spin pack utilizes a relatively thin (i.e., 0.020 inch) stainless steel orifice plate in which a plurality of orifices are cut. The cutting operation is relatively expensive, thereby rendering the orifice plate too expensive to be disposable instead of being periodically cleaned. As noted above, the periodic cleaning and the required post-cleaning inspection are of themselves quite expensive. Further, the density of orifices permitted by the cutting procedure is severely limited. Specifically, the orifice density that can be obtained in the Cheetham orifice plate is no greater than that obtained in the machined distribution plate disclosed in U.S. Pat. No. 4,052,146 (Sternberg) in which the orifice density is 2.93 orifices per square centimeter. Although not disclosed in the Cheetham patent, it is conceivable that one of ordinary skill in the art, armed with hindsight derived from the disclosure of my invention set forth below, might consider the possibility of etching, rather than cutting, the distribution orifices in the orifice plate. To do so, however, would not solve the problem. Cheetham discloses apertures having lengths L of 0.020 inch (i.e., the plate thickness) and diameters D of 0.009 inch, resulting in a ratio L/D of 2.22. For ratios of L/D in excess of 1.50, it is necessary to drill or ream the holes, even if they are initially etched, in order to assure uniform diameters. The drilling/reaming procedure adds a significant cost to the plate fabrication process and, thereby, precludes discarding as an alternative to periodic cleaning of the plate.
It is also desirable that spin packs be useful for both melt spinning and solution spinning. Melt spinning is only available for polymers having a melting point temperature less than its decomposition point temperature. Such polymers can be melted and extruded to fiber form without decomposing. Examples of such polymers are Nylon, polypropylene, etc. Other polymers, such as acrylics, however, cannot be melted without blackening and decomposing. The polymer, in such cases, can be dissolved in a suitable solvent (e.g., acetate in acetone) of typically twenty per cent polymer and eighty per cent solvent. In a wet solution spinning process the solution is pumped, at room temperature, through the spinneret which is submerged in a bath of liquid (e.g., water) in which the solvent is soluble so that the solvent can be removed. It is also possible to dry spin the fibers into hot air, rather than a liquid bath, to evaporate the solvent and form a skin that coagulates.
Molten polymers normally have viscosities in the range of 500-10,000 poise. The polymer solutions, on the other hand, have much lower viscosities, normally on the order of 100-500 poise. The lower viscosity of the solution requires a lower pressure drop across the spinneret assembly, thereby permitting relatively thin distribution plates and smaller assemblies when spinning plural component fibers. Generally, in the prior art, the relatively high orifice packing density (i.e., orifices per square centimeter of spinneret surface) used for low viscosity solution spinning cannot generally be used for the high viscosity melt spinning. As indicated above, it is desirable to have a high orifice density, whether the spin pack is used for solution spinning or melt spinning.
In initially directing the polymer components of different types to appropriate distribution flow paths formed in the distributor plates, it is important that the pressure of the polymer be the same throughout each plane extending transversely of the flow direction. The reason for this is that significant transverse pressure differences prevent the different spun fibers from being mutually uniform. In order to compensate for transverse pressure irregularities that might occur as the polymer is spread over a large area from a relatively small polymer component inlet, the prior art has typically required long distribution apertures in which a high pressure drop is produced to minimize the effect of any lack of pressure uniformity created upstream by the spreading of the polymer flow. The long holes must be drilled, reamed, broached, etc., very accurately in a distributor plate that is relatively thick in order to provide the necessary length of distribution apertures. The thick plate and the accurate machining are both expensive and preclude any realistic possibility of rendering the plates disposable as an option to periodic cleaning. It is desirable, therefore, to provide a distribution plate which is sufficiently inexpensive as to be disposable, with accurate flow distribution paths defined therein, and which functions in conjunction with primary polymer feed slots that minimize pressure variations transversely of the flow direction and upstream of the distribution plate.